uhd: remove external clock polarity setting
uhd: remove external clock polarity setting

Unused and causes the follwing errors on certain UHD versions.

"ValueError: unhandled clock configuration reference source: _external_"

Signed-off-by: Thomas Tsou <ttsou@vt.edu>

git-svn-id: 504cad4147f1e500be0a169b9f46375eaf875953@2664 19bc5d8c-e614-43d4-8b26-e1612bc8e597

telenoobie/osmo-trx,kkroo/osmo-trx,osmocom/osmo-trx,osmocom/osmo-trx,pothosware/osmo-trx,osmocom/osmo-trx,pothosware/osmo-trx,telenoobie/osmo-trx,pothosware/osmo-trx,osmocom/osmo-trx,pothosware/osmo-trx,kkroo/osmo-trx,ttsou/osmo-trx,telenoobie/osmo-trx,telenoobie/osmo-trx,ttsou/osmo-trx,kkroo/osmo-trx,ttsou/osmo-trx,kkroo/osmo-trx,ttsou/osmo-trx